2-Phenylbanzothiazole polymers

ABSTRACT

Mono- and di-acetylenic-substituted 2-phenylbenzothiazoles are thermally self-polymerized to form thermally stable polymers. The polymers are useful as laminating resins for fibrous composites, particularly for high temperature applications.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.

This is a continuation-in-part of application Ser. No. 356,577, filed Mar. 9, 1982, now abandoned.

This invention relates to acetylene-containing aromatic benzothiazole compounds.

In recent years, considerable research has been done in the area of high temperature organic polymers, especially aromatic heterocycles. Among the more thermally stable aromatic heterocycle polymers are the polybenzothiazoles.

The polybenzothiazoles have been synthesized by at least two procedures. First, a fused benzothiazole may be obtained by cyclization of an ortho-mercapto aromatic amine with an acid derivative. This can be accomplished in one step in polyphosphoric acid. Alternatively, the polybenzothiazole can be formed in two steps, the first being condensation to the polyamide in solution, and the second being cyclization of the precursor. Second, the benzothiazole nucleus may be formed prior to polymerization. The latter procedure is disadvantageous because no easily soluble prepolymer is available. A disadvantage common to both of these procedures is that the reaction during curing may produce one or more undesirable by-product, such as an off-gas. What is desired is a polymerization system which will provide a thermally stable polymer, which is easily processable prior to polymerization or during post-curing.

We have prepared a series of compounds that can be thermally polymerized to form thermally stable benzothiazole polymers.

It is an object of the present invention to provide novel benzothiazole polymers and copolymers.

Other objects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following disclosure.

In accordance with the present invention there is provided a 2-phenylbenzothiazole polymer formed by the thermal, self-polymerization of a 2-phenylbenzothiazole monomer having at least one terminal acetylene group of the formula ##STR1## wherein R_(a) and R_(b) are --H or --C.tbd.CH, and wherein at least one of R_(a) or R_(b) is --C.tbd.CH.

Also provided in accordance with this invention is a copolymer formed by the thermal polymerization of a mixture of monomers of the above formula, wherein one of the monomers is mono-acetylenic-substituted and the second monomer is di-acetylenic-substituted.

The mono- and di-acetylenic-substituted 2-phenylbenzothiazoles employed according to the present invention are prepared by reacting the corresponding mono- or di-halogen-substituted 2-phenylbenzothiazole with an alkyne such as 2-methyl-3-butyne-2-ol or ethynyltrimethyl silane in the presence of a catalytic amount of a suitable catalyst. In the case of 2-methyl-2-butyne-2-ol, the reaction involved can be represented by the following: ##STR2##

The adduct III may be mono- or di-substituted, depending upon the degree of substitution of the starting mono- or di-halogen-substituted 2-phenylbenzothiazole. Displacement of acetone by hydrolysis of the mono- or di-butynol adduct III provides the compounds I, as represented by the following: ##STR3##

The mono- or di-halogen-substituted 2-phenylbenzothiazole (II) can be prepared by reacting an o-amino-thiophenol with a benzoic acid. One or both of the reactants may be mono-halogen-substituted, the reaction involved can be represented by the following ##STR4## where X represents --F, --I, or, preferably, --Br.

The reaction of an appropriate o-aminothiophenol with an appropriate benzoic acid may be carried out in polyphosphoric ester and an inert organic solvent such as chloroform, toluene, benzene, or the like, at moderate temperature for 1-3 hours, or it may be carried out in polyphosphoric acid at elevated temperatures (e.g., 200° C.) to yield the corresponding mono- or di-halogen-substituted 2-phenylbenzothiazole.

Treatment of the mono- or di-halogen-substituted 2-phenylbenzothiazole with the acetylenic compound may be carried out in a suitable solvent in the presence of a suitable catalyst. In the case of 2-methyl-3-butyn-2-ol, the solvent may be a basic solvent, such as triethylamine, diethylamine, or the like, including mixtures of these solvents with dimethylacetamide. A suitable catalyst mixture for this reaction includes triphenylphosphine parradium (II) dichloride, triphenylphosphine and cuprous iodide, in a ratio, by weight, ranging from 1:5:2 to 1:10:5, respectively. The amount of this catalyst mixture can range from about 1 to about 10 percent, preferably about 2 to 5 percent, by weight, based upon the weight of the substituted 2-phenylbenzothiazole reactant. The reaction is carried out in an inert atmosphere at a temperature in the approximate range of 75°-100° C. for a period of about 12 to 36 hours. The acetylenic compound is employed in excess, generally from about 1.5 to about 2.5 times the amount required to replace the replaceable halogen in the 2-phenylbenzothiazole compound.

The adduct III is converted to the acetylene-containing compound I by hydrolysis with a strong base, such as potassium hydroxide, sodium hydroxide, lithium hydroxide, or the like, in a suitable solvent, such as toluene. The reaction is carried out in an inert atmosphere under reflux conditions for a period of about 1 to 10 hours.

The acetylene-containing 2-phenylbenzothiazole compounds of this invention are solids at room temperature which can be thermally homopolymerized by heating the monomers in an inert or oxidative atmosphere at a temperature ranging from about 200° to 380° C. A heating period of from about 1 to 2 hours is usually sufficient to obtain a complete cure although longer times, e.g., up to 24 hours, can be used.

The acetylenic 2-phenylbenzothiazole monomers of this invention are soluble in a variety of low boiling organic solvents, such as, for example, methylene chloride, chloroform, ethylene chloride, toluene, xylene, and the like.

The polymers of this invention are useful in various applications wherein thermal stability is a particular requirement. These polymers are especially useful for fabricating high temperature stable laminated structures. For example, one or more of the monomeric acetylene-containing 2-phenylbenzothiazole compounds may be dissolved in one or more of the aforementioned solvents, and the resulting mixture spread onto a high temperature stable film, fiber or cloth, such as woven graphite cloth, to form an impregnated material, or prepreg. A plurality of plies of the prepreg are laid up, heated to remove the solvent, then further heated to polymerize the monomers, as described above, thereby uniting the laminate structure. Alternatively, the solvent may be removed prior to layup of the several plies. Further, after removal of the solvent, the prepreg may be heated to a temperature sufficient to initiate polymerization of the monomer(s), i.e., a temperature of about 120° to 195° C., for a brief period of about 10 to 30 minutes, to partially polymerize the monomer(s) and to adhere the monomer(s) to the substrate. Following such partial polymerization, the prepreg may be quenched to halt further polymerization. The partially polymerized prepreg may be laid up, then cured as described above.

While it is not intended to be limited to any particular theory, in the curing step it is believed that the terminal ethynyl groups propagate to form polyenes. In the case of the mono-acetylenic compounds, B--C.tbd.CH, where B represents a mono-substituted 2-phenylbenzothiazole group, the polymer is a linear polyene having the general structure ##STR5## wherein n has a value of about 1 to 50.

In the case of the di-acetylenic compounds, HC.tbd.C--B'--C.tbd.CH, where B' represents a di-substituted 2-phenylbenzothiazole group, the resulting polymer has a polyene network, believed to be as follows ##STR6## wherein each n has a value in the range of 1 to 10.

The diacetylenic compounds may also be polymerized in admixture with the monoacetylenic compounds, the latter acting as a diluent for the former.

The following examples illustrate the invention.

EXAMPLE I Preparation of 2-(4-bromophenyl)-5-bromobenzothiazole

A mixture of 20 g (0.083 mole) of 4-bromo-2-aminothiophenol hydrochloride and 270 g of deoxygenated polyphosphoric acid was heated at 70° C. for 4 hours. To this mixture was added a solution containing 22 g (0.11 mole) of p-bromobenzoic acid dissolved in 109 g of sulfolane. The reaction mixture was slowly heated to 200° C. and maintained at that temperature for 1 hour. After cooling, the reaction mixture was poured into a 1:1 water-methanol solution to precipitate the product. The light tan precipitate was filtered, washed with dilute ammonium hydroxide, washed with water, and then air dried. The 2-(4-bromophenyl)-5-bromobenzothiazole was recrystallized from toluene to give 24.5 g (80% of theoretical), m.p. 162°-163° C.

Analysis: Calc'd for C₁₃ H₇ NSBr: C, 42.30; H, 1.90; N, 3.90; Found: C, 42.2; H, 1.56; N, 3.81

EXAMPLE II Preparation of 2-(4-Ethynylphenyl)-5-ethynylbenzothiazole

A mixture of 28 g (0.078 mole) of 2-(4-bromophenyl)-5-bromobenzothiazole, 0.09 g of triphenylphosphine palladium II dichloride, 0.7 g of triphenylphosphine, 0.26 g of cuprous iodide and 250 ml of triethylamine was degassed by passing nitrogen through the system for 20 minutes. To the degassed mixture was added 28 g (0.33 mole) of 2-methyl-3-butyne-2-ol. The reaction mixture was heated to 90° C. and maintained at that temperature for 24 hours. After cooling, the precipitated amine hydrobromide was removed by filtration. The filtrate was evaporated under reduced pressure leaving a brown solid. The solid was then dissolved in chloroform. The chloroform solution was washed with a 10% sulfuric acid solution, then with water, and thereafter dried over anhydrous magnesium sulfate. The chloroform was removed under reduced pressure, yielding 27 g (100% of theoretical) of the bisbutynol adduct which was used without purification.

A mixture of 27 g (0.078 mole) of the bisbutynol adduct dissolved in 250 ml of toluene and 2.0 g of powdered potassium hydroxide was heated to reflux. The progress of the reaction was monitored by thin layer chromatography. After about 4 hours, all the starting material was hydrolyzed. The reaction mixture was filtered and the toluene was removed under reduced pressure. The residue was eluted through a silica gel column using methylene chloride as the eluant. Removal of the eluant provided 18.4 g (91% of theoretical) of pure product: m.p. 150°-161° C.

Analysis Calc'd for C₁₇ H₉ SN: C, 78.74; H, 3.50; N, 5.40; S, 12.36; Found: C, 78.43; H, 2.91; N, 5.07; S, 12.03

EXAMPLE III

Using the procedures given in Examples I and II, a series of mono- and diacetylenic compounds were prepared. The melting points and elementary analyses of these compounds are given in Table I below.

                  TABLE I                                                          ______________________________________                                                    m.p., Elemental Analysis                                                       °C.                                                                           C. %    H, %    N, %  S, %                                    ______________________________________                                         Mono-Acetylenic                                                                Compound                                                                       (Calc'd for C.sub.15 H.sub.9 NS)                                                            --      (76.56) (3.85)                                                                               (5.95)                                                                               (13.63)                               2-(3-ethynylphenyl)-                                                                        87      77.93   4.46  6.70  14.32                                 benzothiazole                                                                  2-(4-ethynylphenyl)-                                                                        124     77.27   3.72  5.57  13.20                                 benzothiazole                                                                  2-phenyl-5-ethynyl-                                                                         115     76.57   3.57  5.28  13.61                                 benzothiazole                                                                  2-phenyl-6-ethynyl-                                                                         117     76.65   3.29  5.71  13.26                                 benzothiazole                                                                  Di-Acetylenic                                                                  Compounds                                                                      (Calc'd for C.sub.17 H.sub.9 NS)                                                                    (78.73) (3.50)                                                                               (5.40)                                                                               (12.36)                               2-(4-ethynylphenyl)-5-                                                                      154     78.43   2.91  5.07  12.03                                 ethynylbenzothiazole                                                           2-(3-ethynylphenyl)-5-                                                                      169     78.00   2.85  5.02  11.49                                 ethynylbenzothiazole                                                           2-(4-ethynylphenyl)-6-                                                                      >150    78.56   2.78  5.59  11.38                                 ethynylbenzothiazole                                                           2-(3-ethynylphenyl)-6-                                                                      185     78.42   2.99  5.22  11.93                                 ethynylbenzothiazole                                                           ______________________________________                                    

EXAMPLE IV Polymerization of Mono-Acetylenic Compounds

The four mono-acetylenic 2-phenylbenzothiazole compounds prepared as in Examples II and III were thermally homopolymerized by heating samples of each compound under a blanket of nitrogen for a period ranging from 1 to 2 hours. The polymerization temperature, Tp, was determined by differential scanning calorimetry (DSC) at a heating rate of 10° C./min. The glass transition temperature, Tg, was determined by thermal mechanical analysis (TMA) at a heating rate of 10° C./min following curing of each polymer at 316° C. for 6 hours under a nitrogen atmosphere. The data obtained are shown below in Table II.

                  TABLE II                                                         ______________________________________                                         Thermomechanical Properties of Polymers                                        Derived from Mono-Acetylenic Monomers                                                       m.p.,   Tp, On-  Tp, maxi-                                        Monomers     °C.                                                                             Set (°C.)                                                                        mum (°C.)                                                                       Tg, (°C.)                         ______________________________________                                         2-(3-ethynylphenyl)-                                                                         87     175      246     148                                      benzothiazole                                                                  2-(4-ethynylphenyl)-                                                                        124     135      210     140                                      benzothiazole                                                                  2-phenyl-5-ethynyl-                                                                         115     150      244     172                                      benzothiazole                                                                  2-phenyl-6-ethynyl-                                                                         117     125      204     192                                      benzothiazole                                                                  ______________________________________                                    

EXAMPLE V Polymerization of Di-Acetylenic Compounds

The four di-acetylenic 2-phenylbenzothiazole compounds prepared as in Examples II and III were thermally homopolymerized as described in Example IV. The data obtained are shown below in Table III.

                  TABLE III                                                        ______________________________________                                         Thermomechanical Properties of Polymers                                        Derived from Di-Acetylenic Monomers                                                         m.p.,   Tp, On-  Tp, maxi-                                        Monomer      °C.                                                                             Set (°C.)                                                                        mum, (°C.)                                                                      Tg, (°C.)                         ______________________________________                                         2-(4-ethynylphenyl)-5-                                                                      154     158      197     377                                      ethynylbenzothiazole                                                           2-(3-ethynylphenyl)-5-                                                                      169     175      214     344                                      ethynylbenzothiazole                                                           2-(4-ethynylphenyl)-6-                                                                      >150    150      203     307                                      ethynylbenzothiazole                                                           2-(3-ethynylphenyl)-6-                                                                      185     190      207     339                                      ethynylbenzothiazole                                                           ______________________________________                                    

It will be evident to those skilled in the art that modifications of the present invention can be made in view of the foregoing disclosure without departing from the spirit and scope of the invention. 

We claim:
 1. A 2-phenylbenzothiazole resin formed by the thermal, self-polymerization of a 2-phenylbenzothiazole monomer having at least one terminal acetylene group of the formula ##STR7## wherein R_(a) and R_(b) are --H or --C.tbd.CH, wherein at least one of R_(a) or R_(b) is --C.tbd.CH.
 2. A polymer in accordance with claim 1 wherein said monomer is 2-(3-ethynylphenyl)benzothiazole.
 3. A polymer in accordance with claim 1 wherein said monomer is 2-(4-ethynylphenyl)benzothiazole.
 4. A polymer in accordance with claim 1 wherein said monomer is 2-phenyl-5-ethynylbenzothiazole.
 5. A polymer in accordance with claim 1 wherein said monomer is 2-phenyl-6-ethynylbenzothiazole.
 6. A polymer in accordance with claim 1 wherein said monomer is 2-(4-ethynylphenyl)-5-ethynylbenzothiazole.
 7. A polymer in accordance with claim 1 wherein said monomer is 2-(3-ethynylphenyl)-5-ethynylbenzothiazole.
 8. A polymer in accordance with claim 1 wherein said monomer is 2-(4-ethynylphenyl)-6-ethynylbenzothiazole.
 9. A polymer in accordance with claim 1 wherein said monomer is 2-(3-ethynylphenyl)-6-ethynylbenzothiazole.
 10. A 2-phenylbenzothiazole resin formed by the thermal, self-polymerization of a mixture of a mono-acetylenic monomer of the formula ##STR8## wherein R_(a) is --H or --C.tbd.CH, R_(b) is --C.tbd.CH or --H and only one of R_(a) or R_(b) is --C.tbd.CH, and a di-acetylenic compound of the same formula wherein both R_(a) and R_(b) are --C.tbd.CH. 